usb: cdns3: remove redundant assignment to pointer trb

Pointer trb being assigned with a value that is never read, it is
assigned a new value later on. The assignment is redundant and
can be removed.

Addresses-Coverity: ("Unused value")
Signed-off-by: Colin Ian King <colin.king@canonical.com>
Signed-off-by: Felipe Balbi <balbi@kernel.org>